Clean future energy in ammonia

The Romans named it after Amon-Ra, king of the Egyptian gods, but most people today probably think of it as a kitchen cleaner.

Nevertheless, ammonia looks set to live up to its ancient billing following research that could make it a global source of energy.

Scientists in Britain have found a way to turn this widely available compound into a cheap, clean and safe fuel by splitting it into its two components: nitrogen, and that much vaunted fuel of the future, hydrogen.

Chemical engineers have long been able to “crack” ammonia – formula NH3 – using expensive catalysts. But as with so many potential wonder fuels, the key challenge has been to find a way of doing it economically.

Now a team at the UK Science and Technology Facilities Council (STFC) near Didcot, Oxfordshire, has rediscovered a long-forgotten way of doing just that.

In the process, the technique appears to overcome key barriers to the use of hydrogen, such as storage and transportation.

No one doubts the theoretical advantages of hydrogen as a fuel. It’s the most abundant element in the universe, packs in far more energy than any comparable fuel, while leaving behind nothing more noxious than water.

Yet the practical barriers to its exploitation are formidable. As the lightest of all gases, it’s hard to keep and transport, requiring 7,000 tonnes of pressure per square metre, which requires energy and expensive compression equipment, storage chambers and distribution pipes.

It suffers, too, from an unfortunate and undeserved image as a uniquely flammable gas. These hurdles have stymied the progress of hydrogen as a widely-used fuel.

Ammonia is an altogether less intimidating prospect. In its structure, nature keeps hydrogen under control by binding it to atoms of nitrogen, a virtually unreactive element.

Natural compounds of ammonia drew Romans to the temple of Amon-Ra in ancient Libya. So-called sal ammoniac – ammonium chloride – drew the attention of Abu Musa Jabir ibn Hayyan, a Muslim alchemist, because of its effect on metals.

Only centuries later did chemists realise it was a paradoxical mix of a potent fuel and an unreactive gas.

The tricky bit is exploiting it. The standard approach has been a combination of temperature and expensive catalysts such as metal ruthenium.

But in 1894, Arthur Titherley, a chemist in Liverpool, reported that when ammonia was passed over a hot, salt-like material called sodamide, it split apart into its constituent gases of nitrogen and hydrogen.

Amazingly, the significance of what Titherley realised was “an interesting result” has only now been recognised.

And according to the STFC team that has revived Titherley’s works, the sodamide process may hold the key to unlocking the potential of hydrogen as an everyday fuel.

In experiments reported recently in the Journal of the American Chemical Society, Professor Bill David and his colleagues found that the reaction was at least as good at extracting hydrogen from ammonia as those using catalysts – and potentially far cheaper.

That in turn opens up a whole new view of the storage and transportation of hydrogen by exploiting the already-existing infrastructure for ammonia.

This stems from ammonia’s critical role in another long-standing global challenge: feeding the world. For almost a century it has held off the Malthusian threat of mass starvation through its role in allowing the nitrogen in the atmosphere to be “fixed” by combination with hydrogen, and then turned into fertiliser.

The resulting global demand for ammonia has made it one of the most widely produced and transported bulk chemicals, with handling facilities similar to those of liquid petroleum gas.

So how would this new fuel be used? According to the team, the effectiveness of the sodamide reaction means ammonia could be turned into hydrogen in vehicles.

This could then be turned into electricity via so-called fuel-cell technology – a kind of electric battery that uses hydrogen and atmospheric oxygen to produce energy and water.

While some car makers – notably Toyota, Honda and BMW – are all working on fuel-cell cars for the mass market, concerns about the availability of hydrogen fuel and the problems of onboard storage have long dogged the technology. The sodamide process may change this.

But the STFC team thinks there may be a simpler way forward. They argue that ammonia can be used as a fuel in existing car engines if it is mixed with a small amount of hydrogen created by the sodamide process.

Their calculations suggest that a few litres of ammonia could be enough to run a mid-sized family car. The team is now building a demonstration power unit to investigate the possibilities.

Even if the economics do stack up, there are still some challenges to be overcome before ammonia- powered cars hit the road. Sodamide reacts violently with water and carbon dioxide – the two most common forms of fire extinguisher – while the ammonia it produces is corrosive.

Still, it is virtually a law of physical chemistry that any viable form of energy has disadvantages, and none of those facing ammonia seems insurmountable.

Certainly, if it does fulfil its promise as a fuel, ammonia will be a candidate for the most important industrial molecule in history.

It can already make a strong case. When the First World War broke out exactly a century ago this month, some believed that ammonia-like compounds held the key to restoring peace within weeks.

All the belligerent parties relied on nitrogen-rich compounds to produce explosives, with the biggest reserves being natural resources in South America. When the Allies imposed a blockade on Germany that prevented access to those resources, victory seemed assured.

Unfortunately, the Allies did not take into consideration the Nobel prize-winning brilliance of Fritz Haber, a German chemist who had invented the industrial process for fixing atmospheric nitrogen by creating ammonia.

While it did not give Germany victory, the Haber Process – and the resulting ammonia-based fertilisers – did allow the world to feed itself.

Having given mankind access to its most vital source of energy, ammonia may now be about to do the same for machines.

Robert Matthews is a visiting reader in science at Aston University, Birmingham